Could it be that what you have heard about is the shim's hitting the primary case while installing the M6 and needing to be ground off? If that is what you are referring to, you will know if the shim needs to be ground down when you install it. The shim's will not fit tight against the back of it.
As sleepneagle said follow the instructions. It's pretty simple to install, and doesn't take a lot of time.